Research described in this paper looks at the effect of horizontal density gradient in the vicinity of a river mouth. The model results based on the measured prototype data in Apalachicola Bay, Florida, indicate that horizontal density in the vicinity of a river mouth. The horizontal density gradient terms have been added to a two-dimensional depth averaged model and a two-layer hydrodynamic and salinity model. The salinity results from model with and without the horizontal density gradient terms are presented and compared with prototype data. The statistical analysis of salinity variance at each gage station around the river mouth also demonstrates the effect of the horizontal salinity gradients.